In isotope separation where a single desired isotope species is to be selectively photoexcited and photoionized to permit electro or electromagnetic separation, laser radiation, typically from a dye laser injection locked oscillator, is provided of a frequency matching the excitation frequency of the desired isotope but not corresponding to excitation frequencies of other isotopes. It is known that the absorption spectrum for the desired isotope at any given absorption line is distributed over a predetermined although narrow bandwidth due to hyperline structure, Doppler or Zeeman effects. Accordingly it is desired to provide laser radiation which covers the narrow but nevertheless finite spectrum over which the desired isotope will absorb. This is typically accomplished by exciting a plurality of modes of a laser oscillator such as an injection locked oscillator in the configuration of a ring laser within a bandwidth corresponding to the absorption band of the desired isotope. Further improvements in efficiency can be achieved by frequency sweeping the plurality of modes at least one mode spacing to achieve complete spectral coverage of the absorption band.
In order for such a system to operate effectively it is first necessary to excite all of the modes over which an injection locked oscillator may oscillate within the specific bandwidth defined by frequency selective elements within the injection locked oscillator. In order to excite all modes of the injection locked oscillator it would normally be necessary to apply a multi-mode radiation to the oscillator with each mode matching a mode of the injection locked oscillator. Such a task becomes particularly difficult when a large number of modes are to be excited within the injection locked oscillator.